This invention relates to a multiplying digital-to-analog converter for converting an n-bit-wide digital signal into an analog signal. The converter comprises a R-2R chain network, whose series resistors and whose shunt resistors positioned at either one of the two ends of the chain network each have a resistance R and whose remaining shunt resistors have a double resistance 2R, in which a supply voltage is impressed on the shunt resistor ends remote from the series resistors and the shunt resistor ends near to the series resistors form connection points, and which has an output presenting the output signal.
Such converters are designated multiplying digital-to-analog converters when the amplitude of the analog output signal depends on the magnitude of a current applied to the converter. In a converter comprising a R-2R chain network this current is switched to several connection points of the chain network, so that appropriate voltages and currents occur at the output in response to a digital signal applied to the converter and to be converted to the analog range. If the magnitude of this external current applied to the digital-to-analog converter is additionally varied, this also has an effect on the amplitude of the converter output signal.
Multiplying digital-to-analog converters of this type may thus be used for adjusting the amplitude of an AC signal which is superposed on the current applied to the converter. Depending on the digital word, the amplitude of this AC signal also is then changed in accordance with the states of the bits of the digital word. In addition, also the current, however, which is superposed by the AC current, is changed in accordance with the states of the bits. Thus, at the output of the converter appears not only the AC current whose amplitude is changed, but also a voltage or current value which corresponds to the conversion of the constant current in accordance with the states of the bits of the digital word to the analog range. As it were, a kind of DC current or DC voltage is thus superposed on the AC current or AC voltage signal respectively. This signal will be denoted offset signal hereinafter.
In many applications the time-dependent changes of such offset signals are undesired, because, for example, for adjusting a volume signal by means of such a multiplying digital-to-analog converter, such voltage or current jumps are undesired because they become audible.